Many vehicle collisions occur every year, often causing bodily injury and extensive property damage. Some of these collisions result from inattentive drivers who fail to stop quickly enough when traffic stops. Particularly dangerous conditions exist at night, when drivers are more prone to fatigue and the ability to judge distances is impaired. The ability to judge distance depends, in part, on spatial clues, many of which are obscured by darkness. Adverse whether conditions may similarly obscure spatial clues and impair depth perception. Additionally, congested traffic, with its typical stop and go character, and close vehicle proximities, requires the driver to maintain a constant level of heightened alertness. Even a momentary lapse in attention can result in an collision.
In situations where collisions are inevitable, some automotive systems can be configured to minimize the potential for injury and loss of life. The airbag is an example of one such system. If the type and severity of the collision can be predicted, even to a first approximation, before the collision actually occurs, the airbags can be configured for optimal response. Parameters subject to configuration may include the rate and extent of airbag inflation.
To reduce the seriousness and number of collisions resulting from operator error, ranging sensors have been employed to collect external data and to provide timely warnings to vehicle occupants. Most ranging sensors utilized in collision avoidance include a transmitting portion and a receiving portion. The transmitting portion sends a signal from the sensor-equipped vehicle, or host vehicle, to a target vehicle. The target vehicle serves as a reflector, returning a portion of the transmitted signal to the receiving portion. The delay between the transmission and the reception of the reflected signal provides data pertaining to inter-vehicle distance and relative vehicle dynamics. This type of sensing system will be termed an interrogation/reflection system herein, and usually comes in one of two general types; either a radar-based system that transmits and receives radio waves, or a laser-based system that transmits and receives coherent light instead of radio waves. Both radar and laser-based systems are very costly and, as such, are not affordable to many consumers. Additionally, both systems have certain drawbacks. For instance radar-based interrogation/reflection systems need to be monitored and periodically maintained. A poorly maintained transmitting element, or mismatched antenna, may result in a portion of the transmission signal being reflected back into the transmitter, potentially causing damage. Electromagnetic pollution is another shortcoming common to most radar-based interrogation/reflection systems. There are a finite number of radio frequencies available, and as the number of frequency-requiring devices increases, so does the likelihood of false alarms caused by spurious signals originating from devices using neighboring frequencies or by inadequately shielded devices operating on distant frequencies, but manifesting harmonics within the operational frequencies of the receiving apparatus. Laser-based systems have attempted to overcome the problems associated with the overcrowded radio spectrum by using coherent light instead of radio signals. Although laser-based systems sufficiently overcome some of the problems associated with radio-based signals, they have other significant limitations. For example, precise mounting and alignment, while required in many interrogation/reflection systems, are especially important in laser-based systems. Failure to properly align a laser can result in the transmitted signal either being dissipated in space, or reflecting off an unintended object. Furthermore, lasers, because of their characteristic coherent nature are dangerous if directed into the eye. The risk is most acute with higher-powered lasers, or lasers operating outside of the visible spectrum.